1. Field of the Invention
The present invention relates to stepped mould inserts with which stepped microstructure bodies are produced, and to methods of producing the stepped mould inserts and stepped microstructure bodies. One object of the present invention is to produce high-precision stepped microstructure bodies economically.
2. Discussion of the Background
Microstructure bodies produced by the LIGA method generally have microstructures having lateral dimensions in the micrometer range, accompanied by a height which may be several hundred micrometers. In general, the microstructure bodies are planar; i.e., the lateral dimensions of the microstructures are virtually constant over the entire height of the structure, and the side walls of the microstructures are parallel to one another.
For many applications, it is necessary to produce three-dimensional microstructures, but this is possible in this method using parallel irradiation only to a limited extent by inclining the specimen to the beam path.
Three-dimensional structures can in most cases be produced by a plurality of levels of planar structures having different structural height. This can be achieved by, for example, stepped construction on the baseplate.
According to EP 184 608, bodies having columnar microstructure can be produced, which have different heights in two regions. For this purpose, a layer of a plastic (resist material), the properties of which can be altered by X-ray radiation, is first partially irradiated with X-ray radiation through a patterned mask, in which process the entire thickness of the resist layer is penetrated by the X-ray radiation. The resist layer is then partially irradiated again with X-ray radiation through a second patterned mask, in which process the penetration depth of the radiation is less than the thickness of the resist layer and the irradiated regions are larger than in the case of the first irradiation. This process is in principle suitable for producing microstructure bodies having more than two different structural heights. A separate irradiation step is necessary for each structural height. For each irradiation step, use is made of a separate mask, which has to be positioned with high precision relative to the already irradiated regions of the resist layer. This appreciably increases the cost and noticeably decreases the yield. The precise step height, a smooth step level and a precise orthogonality with respect to the sides of the microstructure are difficult to establish.
EP 253 066 provides a method of producing microstructure bodies having regions with different structural heights. A resist layer is irradiated once through a single mask, the structure of which corresponds to the structure of the microstructure body (absorber structure) (X-ray depth lithography). The mask is composed of a patterned layer which absorbs the X-ray radiation almost completely (total absorber layer), and at least a further patterned layer which only partly absorbs the X-ray radiation (partial absorber layer). The resist material is a plastic having a sharp lower limit dose (for example, polymethyl methacrylate). For the partial absorber layers, substances are used which have different absorptive powers within the wavelength range of the X-ray radiation used. The nature of the partial absorber materials, their respective layer thickness, the spectrum of the X-ray radiation used and the radiation dose have to be matched to the desired different structural heights of the microstructure body. This method requires extensive calculations. In the case of this method, too, the precision of the microstructure is limited.
DE 41 42 001.2 describes the production of stepped mould inserts by combining mechanical precision machining and X-ray lithography. Owing to the machining methods, a finite precision of the step dimensions (limited to more than 10 micrometers) is achievable, particularly in relation to the stepslope area and the step height.
U.S. Pat. No. 5,190,637 teaches a method for the formation of microstructures by multiple level deep X-ray lithography with sacrificial metal layers. At first the voids in an exposed and developed photoresist layer are filled with a primary metal by electrodeposition. After removal of the remaining parts of the resist layer the thus created voids are filled with a secondary metal. This layer consisting of two different metals is machined down to a predetermined thickness. This series of method steps is--optionally multiply--repeated. The masks used for exposing the resist layers have patterns different from each other. Finally, the secondary (sacrificial) metal is removed by an etchant which does not etch the primary metal. The microstructure body formed is composed of several layers of the primary metal, said layers being formed during several steps. An adhesion promotor is optionally applied. It is well known that a microstructure body formed by such method is sensitive to high thermal and mechanical stress.
U.S. Pat. No. 4,351,653 teaches a method for producing separating nozzle elements by lithographically structuring a resist layer and electroplating a metal in the developed layer. These steps are multiply repeated to achieve a predetermined height of said structure. Each resist layer is exposed using the same mask. The metallic microstructure body is composed of several layers of said metal. The variation in thickness of said metal layers is not of any importance. The metallic microstructure body obtained after a final machining step has the same thickness at any point, consequently such body has no steps. It is well known that even said microstructure body is sensitive to high thermal and mechanical stress.
In summary, stepped mould inserts have been produced using patterned irradiation masks having locally varying absorption of X-ray radiation. For this purpose, extensive calculations are necessary. Furthermore, a plurality of masks may be used consecutively, and the penetration depth of the X-ray radiation may change for each irradiation step. These masks have to be positioned with high precision.
Consequently, a simple method of producing high-precision stepped mould inserts with which high-precision stepped microstructure bodies can then be moulded is still felt.